I. Field of the Invention
This invention generally relates to the electrodeposition of gold or gold alloys onto base metals.
II. Description of the Prior Art
The electrodeposition of gold on base metals or base metal alloys can readily be accomplished by known methods and are generally satisfactory for most purposes. When these gold plated base metals, however, are subjected to high temperatures for short periods of time (e.g. 450.degree. to 500.degree.C for 5 to 10 minutes) or at more moderate temperatures for longer periods of time (e.g. 300.degree.C for 70 hours), the base metal diffuses into the gold plating which adversely affects the desired function of the gold plating such as electrical conductivity, often to the point of complete failure of the gold plated component for its intended function. This diffusion can generally be visually observed by a discoloration of the gold plating.
One manner of overcoming this diffusion problem, which has been practiced in the art, is to electrodeposit extremely thick layers of gold, as high as 80 microinches or higher so that the diffusion of the base metal into the gold layer does not significantly affect the outer surface or the functional portion of the gold deposit so that the gold plated component can function for its intended purpose at high temperatures and time. The use of such extremely thick deposits of gold is of course very expensive and in many cases is not practical or useful in commercial operations.
Various barrier coatings have been used in the past in an attempt to prevent or inhibit this diffusion of the base metal into the gold platings to reduce the thickness of the gold deposits and thereby reduce the cost. For example, chromium has been suggested as a barrier coat to prevent diffusion of the base metal into the gold layer, but this has never been commercially practical particularly in view of the significant difficulties encountered in attempting to plate gold over chromium metal with present technology and the difficulties encountered in plating chromium in barrels to obtain sufficient chromium coverage over the surface of the large variety of parts encountered in electronic component plating.
Nickel barrier coats have also been used to prevent diffusion of base metal into the gold. Although nickel barrier layers have been partially successful, fairly thick deposits of gold are still required. Gold plated components having a nickel barrier thereon can generally stand about 500.degree.C. for about 5 to 10 minutes provided that the thickness of the gold deposits is on the order of 60 to 70 microinches. This is of course an improvement over the deposition of gold directly on the base metal which requires about 80 microinches under the same operating conditions to prevent diffusion of the base metal and possible failure. Nickel barrier coatings also introduce into the gold plated components, magnetism which may impair the usefulness of the gold plated components particularly where the magnetic effect of the nickel coating would interfere with the proposed electronic function of the component.